1. Field of the Technical Field
The present invention relates to an apparatus for regulating state of charge in a battery assembly, and more particularly, to an apparatus for regulating state of charge in a battery assembly which includes secondary cells as a plurality of unit cells connected in series.
2. Description of the Related Art
In an electric car driven by an electric motor and in a hybrid electric car driven by both an engine and an electric motor, a battery assembly including a plurality of secondary cells as unit cells connected in series, such as a nickel-metal hydride battery or a lithium battery, has been employed as an electric source for the electric motor.
It has been known that the above-described battery assembly has had such a problem that due to repeated charging and discharging of electricity, variations may occur in respective voltages across the unit cells depending on the state of charge (SOC) of the unit cells, and in the case where charging and discharging of electricity are repeated while the variations remain unsolved, some of the unit cells may fall in an excessively charged state or in an excessively discharged state.
Under the circumstances, a discharge type apparatus for regulating state of charge, which can solve variations in respective voltages across a plurality of unit cells by extracting a minimum unit cell having a minimum voltage from the plurality of unit cells, sequentially connecting the unit cells except the minimum unit cell to a discharge resistor, and discharging stored electric charge until the respective voltages across the connected unit cells are equal to the voltage across the minimum unit cell, is proposed (for example, Japanese Unexamined Patent Application Publication No. 2002-199510).
In addition, a charge pump type apparatus for regulating state of charge, which can solve variations in the respective voltages across a plurality of unit cells by transferring electric charge from a unit cell having a high voltage to a unit cell having a low voltage through a capacitor is proposed. As an example of the charge pump type apparatus for regulating the state of charge, it is proposed that unit cells are sequentially connected to a capacitor (Japanese Unexamined Patent Application Publication 1998-225005). In this structure, electric charge of the unit cell which has a higher voltage than a voltage across the capacitor is transferred to the capacitor, and contrarily, electric charge of the capacitor is transferred to the unit cell which has a lower voltage than the voltage across the capacitor. In other words, the stored electric charge is transferred from the unit cell having the higher voltage to the unit cell having the lower voltage by way of the capacitor, and consequently, the variations in the respective voltages across the unit cells can be eliminated.
It is proposed that a capacitor is alternately connected to a maximum unit cell having a maximum voltage and a minimum unit cell having a minimum voltage (Japanese Unexamined Patent Application Publication No. 2004-120871). In this structure, since the electric charge is repeatedly transferred from the maximum unit cell to the minimum unit cell, the respective voltages across the unit cells can be equalized in a short time. Since the capacitor has a very small internal resistance, connection between each of the unit cells and the capacitor is necessarily performed by a current limiting resistor, the current limiting resistor limits charging and discharging current which flows in the unit cells and over-current is prevented from flowing.
However, in the discharge type apparatus for regulating the state of charge, the discharge resistor having one resistance value is connected regardless of the sizes of the voltages across the unit cells. The resistance value of the discharge resistor is determined to a value for allowing discharge current equal to or less than allowable current to flow when a unit cell having a full-charged state is connected to the discharge resistor. The allowable current is determined according to the characteristics of the secondary cell included as the unit cell and the secondary cell may deteriorate when the discharge is performed by current exceeding the allowable current.
Accordingly, when the voltage across the unit cell is relatively large, discharge current which flows from the unit cell to the discharge resistor is high and the voltage across the unit cell becomes the voltage across the minimum unit cell. In contrast, when the voltage across the unit cell is small, the discharge current which flows from the unit cell to the discharge resistor decreases and it takes a considerable time to regulate the voltage across the unit cell to the voltage across the minimum unit cell. Thus, as the voltage across the unit cell decreases, it takes a considerable time to equalize the voltages across the plurality of unit cells.
In the charge pump type apparatus for regulating the state of charge, the current limiting resistor having one resistance value is connected regardless of a difference between a voltage across the unit cell and a voltage across the capacitor. Accordingly, when the difference between the voltage across the unit cell and the voltage across the capacitor is large, charge/discharge current which flows between the unit cell and the capacitor is high and a large amount of charge can be transferred in a short time. In contrast, when the difference between the voltage across the unit cell and the voltage across the capacitor decreases by equalization, the charge/discharge current which flows between the unit cell and the capacitor decreases and thus it takes a considerable time to transfer the electric charge. As the results, as the difference between the voltage across the unit cell and the voltage across the capacitor decreases, it takes a considerable time to equalize the respective voltages across the plurality of unit cells.